1. Field of the Invention
The present application relates to an electronic device, and more particularly to a field effect transistor.
2. Description of Related Art
Recently, III-V compound materials, such as GaN, have been developed rapidly in high power devices. The III-V compound materials allow two-dimensional electron gas (2DEG) to generate in a junction by a piezoelectric effect. Due to the high speed, high density and low conductive resistance of electrons, the III-V compound materials have advantages of high power output, high voltage operation and low switching loss.
According to prior researches, it is found that a field effect transistor with a C-doped GaN layer serving as a buffer layer may have a high breakdown voltage if a thinned channel layer (or called a spacing layer) is disposed therein.
However, referring to FIG. 1, the experimental data shown in FIG. 1 are obtained by using a C-doped GaN layer with a C-doping concentration of about 1E18 cm−3, growing intrinsic GaN (i-GaN) layers with different thicknesses as channel layers on the C-doped GaN layer, forming an Al0.25GaN0.75 layer with a thickness of about 20 nm, and then measuring an electron mobility thereof. Then it is found that when the channel layers are thinner, there is a problem that the electron mobility decreases because carbon ions in the C-doped GaN layer diffuse into the region of two-dimensional electron gas (2DEG) between the i-GaN layers and the Al0.25GaN0.75 layer, which reduces the electron mobility.